A. Field of the Invention
The present invention generally relates to a method of producing ferroelectric films (e.g., poly(vinylidene fluoride) (PVDF) based films) and memory devices through a controlled two-step thermal annealing process.
B. Description of Related Art
Memory systems are used for storage of data, program code, and/or other information in many electronic products, such as personal computer systems, embedded processor-based systems, video image processing circuits, portable phones, and the like. Important characteristics for a memory cell in electronic device are low cost, nonvolatility, high density, writability, low power, and high speed. Conventional memory solutions include Read Only Memory (ROM), Programmable Read only Memory (PROM), Electrically Programmable Memory (EPROM), Electrically Erasable Programmable Read Only Memory (EEPROM), Dynamic Random Access Memory (DRAM) and Static Random Access Memory (SRAM).
More recently, ferromagnetic RAM (FRAM) has been attempted. FRAM utilizes a ferromagnetic region or film of a ferroelectric capacitor, thin film transistor, or diode to generate a nonvolatile memory cell. Such electronic devices are fabricated using two parallel conductive plates separated by a ferroelectric polymer layer. The ferroelectric polymer layer is a layer of insulating film which contains a permanent electrical polarization that can be reversed repeatedly, by an opposing electric field. As a result, the ferroelectric capacitor, thin film transistor, or diode has two possible non-volatile states, which they can retain without electrical power, corresponding to the two binary logic levels in a digital memory. Additionally, ferroelectric capacitors, transistors, and diodes also provide energy-storing functionality. When a voltage is applied across the plates, the electric field in the ferroelectric material displaces electric charges, and thus stores energy. The amount of energy stored depends on the dielectric constant of the insulating material and the dimensions (total area and thickness) of the film.
Typically, poly(vinylidene fluoride) (PVDF) type polymers or copolymers (e.g., a copolymer of PVDF with trifluoroethylene (TrFe) (PVDF-TrFe)) are used as the ferroelectric material due to their large polarization values and electrical and material properties. PVDF type polymers are attractive for electronic devices as they can be produced in the form of films and in a variety of shapes, have high chemical resistance, and high efficiency in converting mechanical energy to electrical energy. PVDF has five different polymorphs (also referred to as phases), alpha (α), beta (β), gamma (γ), delta (δ) and epsilon (∈), with the most common of the polymorphs being the alpha (α) polymorph. The alpha polymorph demonstrates little to no ferroelectric properties, while the remaining phases demonstrate stronger ferroelectric properties, with the beta-polymorph being most preferred.
Many attempts have been made to transform the alpha-polymorph to the more desirable polymorphs using various techniques. Two problems, however, continue to arise with the currently available processes. For one, after deposition and annealing of PVDF using solution processes (e.g., spin-coating), the resulting PVDF film oftentimes demonstrates a para-electric (α) phase rather than the desired (β)-phase. Second, and also after annealing, nano-size cracks routinely develop on the PVDF film, which are detrimental to the film's stable operation under applied voltage.
For instance, Kang in Applied Physics Letters, 2008, Vol. 92, pp. 012921-3 describes a 1-step rapid annealing process at 150° C. The resulting films, however, exhibited a micropattern of (α) and (β) PVDF crystals, confirming a less than desired transition process.
Chinese Application Publication No. CN 103113602 and U.S. Pat. No. 6,514,835 each attempt to address the para-electric (α)-phase/(β)-phase transition by applying pressure or stress to PVDF material during the annealing process, thereby complicating film formation process.
U.S. Pat. No. 8,120,082, by comparison, attempts to solve the phase transition problem through a heating and cooling step. In particular, the PVDF material is heated to a first temperature, which is followed by continuous cooling to an established temperature to effect (β)-phase of the ferroelectric film, which is then followed by rapid cooling (60° C. to 70° C.) so that the film is fixed in the (β)-phase. No attempt, however, is made to solve the problems associated with nano-size crack formation in the resulting films.